The present invention generally relates to downhole valve apparatus used in subterranean well completions and, in a preferred embodiment thereof, more particularly relates to a specially designed combination well casing pressure relief and kill valve.
In an initially completed subterranean well the bore hole is typically lined with a tubular cemented casing having perforations formed therein at a subsurface production zone to admit pressurized production fluid, such as oil and/or natural gas, into the interior of the casing. To flow the admitted production fluid upwardly to the surface of the well, a production tubing string, having an outer diameter smaller than the inner diameter of the casing, is coaxially inserted downwardly into the casing and connected at its lower end to a packer structure positioned above the casing perforations.
The packer seals off the interior portion of the casing below the packer from the annular space between the casing and production tubing above the packer, while at the same time communicating the interior of the production tubing with the casing interior below the packer. To provide hydraulic support for the casing, the casing annulus above the packer in the initially completed well is typically filled with a brine solution. When production flow from the completed well is started, the pressurized production fluid enters the casing through its perforations and flows upwardly through the packer and the production tubing string to the surface of the well for retrieval.
As this initial production continues, the relatively elevated temperature of the upwardly flowing production fluid transfers heat, via the production tubing string, to the sealed-in brine solution within the casing annulus. This production fluid heat transferred to the brine correspondingly increases its pressure, thereby materially increasing the laterally outwardly directed hydraulic force exerted by the brine on the casing. In order to prevent this increased pressure from damaging the casing, it is necessary to relieve the temperature-elevated brine pressure.
A conventional method of automatically providing for this brine pressure relief as the need arises is to install in a subsurface portion of the production tubing string a receptacle, commonly referred to as a side pocket mandrel, that operatively and removably receives a pressure relief valve. The mandrel-received pressure relief valve has an inlet that communicates with the casing annulus via side wall inlet ports in the mandrel, and an outlet communicated with the interior of the mandrel and thus with the interior of the production tubing string. When the casing annulus pressure increases to a level above the set point pressure of the valve, the valve opens to admit pressurized brine from the casing annulus into the production tubing string until the casing annulus pressure falls below the valve set point pressure, whereupon the valve closes to reseal the now lower pressure brine within the casing annulus. This pressure relief valve can also be used in a chemical injection application in which a selected chemical solution is pumped into the casing annulus for introduction into the tubing string via the valve.
Later in the production life of the well it typically becomes necessary to stimulate or otherwise work over the well due to a reduction in its production fluid flow rate caused, for example, by an obstruction of its casing perforations. In order to perform this repair work it is necessary to temporarily "kill" the well--i.e., to temporarily terminate the inflow of production fluid into the casing beneath the packer. The eventual necessity of performing this kill function is typically prepared for in advance by initially installing in the production tubing string a second side pocket mandrel that operatively and removably receives a kill valve. The kill valve has an inlet that communicates with the casing annulus via side inlet ports in its associated mandrel, and an outlet communicated with such mandrel and thus with the interior of the production tubing string.
When it becomes necessary to temporarily kill the well, a kill fluid (such as brine or drilling mud) is pumped into the casing annulus. The continuous pressurized inflow of kill fluid into the casing annulus elevates the annulus pressure until the kill valve pressure set point is reached, at which point the kill valve opens to admit a sustained flow of kill fluid into the production tubing string. The entering kill fluid travels upwardly through the production tubing string and eventually forms a standing kill fluid column which hydraulically prevents upward flow of production fluid through the tubing string.
To provide in advance for these necessary pressure relief and kill functions in an initially completed subterranean well, as described above, it has heretofore been necessary to install in the production tubing string two separate mandrels and a separate valve within each mandrel. This installation redundancy, of course, appreciably increases the overall fabrication cost of the completed well--particularly where the well is constructed offshore. It is accordingly an object of the present invention to provide, in a well completion, these pressure relief and kill functions in a more cost effective manner.